ABSTRACT
Due to a unique combination of electrophysical, chemical, mechanical, and thermodynamic properties, metal oxides can be used in the development of all types of humidity sensors, including ionic, electronic, electrical, and solid-electrolyte-type sensors. To understand the principles of operation of such sensors, in this chapter we analyze various humidity-sensing mechanisms that have been identified for metal-oxide ceramics. In particular, the various stages of water adsorption, including multilayer water adsorption, BET isotherm, pore condensation, and conduction through a “hop-turn” mechanism, first suggested by Grotthuss, are discussed in this chapter. A significant place in the chapter is also occupied by the analysis of approaches used to optimize metal-oxide humidity sensors. It is shown that in order to achieve the best parameters of humidity sensors, it is necessary to optimize porosity, surface area, pore size, chemical composition, film-thickness morphology, grain size, and surface reactivity of metal oxides. The difficulties encountered when using metal-oxide-based nanocomposites are also considered.
